I watched videos on this before- some brands are just highly filtered tap water, some are actually sourced from special springs. There can be a taste difference- my mom insists Arrowhead is 'dusty', and I hate Lifewater (it tastes chemical-y to me).
Having said that- buying water is an extreme rarity for me. It normally only happens on road trips, or if I meant to bring a reuseable bottle somewhere & forgot. When it does come up, usually the only two factors are cost and cap style- I don't want to spend a lot, and I am going to refill that bottle at some point. I avoid the 1-2 brands I know I don't like, otherwise I don't care.